The present invention relates to a magnetic resonance imaging system that calculates shim values and corrects inhomogeneity of static magnetic field based on these values and a method for setting shim values.
An MRI apparatus applies a magnetic field to a subject and produces a tomographic image of the subject, based on detected echo signals from the subject. If the static magnetic field is not homogeneous, phase rotation occurs due to this and causes ghosting and positional distortion (artifacts); it is thus required to keep the static magnetic field homogeneous in order to accurately reflect the internal structure of a probed portion of the subject.
For the MRI, shimming (correcting inhomogeneity of static magnetic field) is important to reduce such artifacts.
When shimming is carried out, conventionally, shim values which are parameters for shimming are calculated based on data acquired by scanning the planes (e.g., x, y, and z planes) based on a system-inherent coordinate system (e.g., a system of x, y, and z coordinates). However, this method has a drawback in which, when slice planes (including tomographic imaging data) are slanted with respect to the system-inherent coordinate system, acquired shim values are non-optimal and it is unable to well correct inhomogeneity.